Monday, December 3, 2012

In my last post, I showed how you could implement the Int construction over resumptions, to derive a higher-order programming language from a first-order one. However, this is not yet a practical tool. The code I gave presented the language as a bunch of combinators, which rapidly get quite painful to write serious programs. Most of us prefer the lambda calculus to SKI combinators for good reason --- programs with variables and binders are much easier to read, since the code that actually does work in not obscured in a rat's nest of plumbing combinators that route data from one combinator to another.

I had hoped to rectify that in this post, by showing how to compile the linear lambda calculus into combinator terms. But when I started
to write it, I realized that I needed to write a warmup post first, which would introduce the basic ideas in a simpler setting. So I'll defer compiling linear types for a little while, and show how to embed total functional programming into OCaml.

Wednesday, November 28, 2012

In this post, I'll show how to turn the Geometry of Interaction construction --- a model of linear logic --- into a runnable Ocaml code. . The basic idea behind this paper is that we can view higher-order behavior, as a disciplined mode of use of a first-order language with recursion --- basically, if you fix a calling convention, then you can compile first-class functions directly and compositionally into a first-order program.

This is interesting because there are a lot of combinator libraries out there which can be seen as DSLs with a natural notion of "first-order program", but which lack a good notion of "higher-order program". (For example, most Haskell libraries based on arrows qualify.) As a programming model, this is very unsatisfying, since this restriction basically says that the author of the library can build interesting abstractions, but clients of the library can't. Wouldn't it be nice if there were a systematic way to turn any first-order language (with a bit of structure) into a higher-order one?

That's exactly what the G construction (also called the Int construction) does. It says that if you have a first-order language with sequential and parallel composition, plus feedback, then you've really got a higher-order language. Stated more formally, in categorical jargon, it says that there is a way of constructing a monoidal closed category from any symmetric monoidal category with a trace operator. (Monoidal products mean parallel composition, categories give you sequential composition, and trace is how category theorists pronounce recursion.)

First, let's get some Ocaml preliminaries out of the way. I'm just
defining function composition and empty and sum types, and their
operations. These should really be in the Ocaml Pervasives
environment, but they aren't. The Haskell prelude is nicer in this regard.

Our running example will come from dataflow and visual programming languages like Simulink. These languages have a model of programming in which you have a bunch of stateful components (adders, multipliers, accumulators), which are connected together by means of wires. This is naturally a first-order formalism, in that it really only makes sense to send data over the wires --- you can't send components over the wire. One way of modeling these kinds of stateful components is by means of resumptions.

A resumption from $I$ (read "input") to $O$ (read "output") is essentially an element of the solution to the domain equation $R = I \to (O \times R)$. You can think of a gadget of this type as something which takes an input, and then gives you an output, plus a new resumption to use on the next input. So it's a bit like a state machine, but formulated in such a way that you don't have to give an explicit state set. It's easy to turn this domain equation into a recursive ML type:

moduleResumption=structtype('i,'o)r=Rof('i->'o*('i,'o)r)

The identity resumption just echoes its input as its output.

(* val id : ('a, 'a) r *)letrecid= R(funx->(x, id))

To compose two resumptions $f$ and $g$, we just feed an input into $f$, and take $f$'s output as the input into $g$.

Note that with composition, we have a category $R$, the category of resumptions. The objects are types, a morphisms $f : I \to O$ is a resumption taking $I$ inputs and producing $O$ outputs.

This category is also "monoidal closed", with the tensor product $A \otimes C$ getting defined as the sum type $A + C$. The intuition is that if we think of $A$ and $C$ as message types, we may want to consider how to react to $A$ and $C$ messages simultaneously. In particular, if we have two machines $f : A \to B$ and $g : C \to D$, we can combine them by taking an $A+C$ message and dispatching to either $f$ or $g$ respectively.

We can also implement feedback --- if we have an $f : A \otimes B \to C \otimes B$, then we can construct a map $A \to C$ by feeding $f$ the $A$, and repeatedly sending the output back into the input until $f$ coughs up a $C$ rather than a $B$:

Now, we can also implement various associativity and commutativity properties. The names are not particularly great (they are just the standard names for the natural transformations associated with a tensor product in category theory), but all the real action is in the types.

Now, we give the G construction in terms of the resumption module we've just defined. In the G construction, we build a new category $G(R)$ from the old one we've just defined. The basic idea here is to talk about bidirectional communication, and so for objects in our new category $G$ will be pairs of objects from our old one.

Objects of $G$ will be pairs $(A^+, A^-)$, where you can think of $A^+$ as a type of messages sent out, and the type $A^-$ as the type of messages that are accepted.

A morphisms of $G$, $f : (A^+,A^-) \to (B^+, B^-)$ will be resumption of type $A^+ \otimes B^- \to A^- \otimes B^+$. Note that the polarities of $A$ and $B$ are flipped in this mapping
-- we are transforming $A$ messages out to $B$ messages out, and we need to accept $B$ messages in order to transform them into $A$ messages in.

moduleG=structopenResumption

Now, let's define the type of morphisms in $G(R)$. Since we don't have type-level pairing in ML, we end up having four type parameters. We use a dummy G constructor to get the type system to produce more useful inferred types.

Composition is trickier. If we have a map $f : (A^+,A^-) \to (B^+, B^-)$ and $g : (B^+,B^-) \to (C^+,C^-)$, we can compose it by feeding $f$'s $B$-output into $g$'s input, and vice-versa, using the trace operator. It's best visualized with the following diagram:
Layer 1fgAABBBBCC++++----

Once we have composition, we can define a tensor product on the $G(R)$ category as well, so that
$(A^+,A^-) \oplus (B^+,B^-) = (A^+ + B^+, A^- + B^-)$. Again, implementing the functorial action requires
implementing the associativity maps.

One cool thing about this category (which leads to the coolest fact about it) is that it naturally supports a dualization, which sends $(A^+,A^-)^\ast \triangleq (A^-,A^+)$. This also has an action on morphisms, which flips around the direction, so that from $f : (A^+,A^-) \to (B^+,B^-)$ we can get $f^\ast : (B^+,B^-)^\ast \to (A^+,A^-)^\ast$.

Now, here's what I consider the neatest fact about the Int construction -- it turns a traced monoidal category into a monoidal closed category. So we can define the linear exponential $(A^+,A^-) \multimap (B^+,B^-) \triangleq (A^+,A^-)^\ast \otimes (B^+,B^-)$.

Wednesday, November 21, 2012

In my previous post, I mentioned limit-colimit coincidence, and how
the ability to define fixed points makes that possible. It turns out
there's a very pretty way of using parametricity to encode this. Now,
inductive and coinductive types are definable in System F as follows:

μα. F(α) = ∀α. (F(α) → α) → α
νβ. F(β) = ∃β. β × (β → F(β))

It's possible to show using parametricity that these type definitions
are actually the inductive and coinductive types we expect, whenever
F is a positive type. That is, a type constructor F
is positive whenever we have a function:

map : ∀α,β. (α → β) → F(α) → F(β)

with the expected properties that:

map [A] [A] id = id

(map [A] [B] f); (map [B] [C] g) = map [A] [C] (f; g)

In lattice theory, you can apply the Knaster-Tarski theorem to get
a fixed point of a function f : L → L when L is a complete
lattice and f is monotone. Intuitively, you can see map
as the proof that the functor F is monotone (really, functorial)
in the lattice (really, category) of types.

It turns out that in a temporal setting, we also need to specify
that we can invoke the algebra/coalgebra operations at multiple
times, which we can specify using the modal stability connective
□A, which changes the definitions to:

μα. F(α) = ∀α. □(F(α) → α) → α
νβ. F(β) = ∃β. β × □(β → F(β))

Now, suppose we have a Nakano style recursive type μ̂α.F(•α), where
every occurrence of the recursive type occurs beneath a delay modality.
Then, assuming that F is positive, we can show how to embed elements of
νβ. F(•β) into elements of μα. F(•α) as follows:

This suggests that guarded term-level recursion is enough to make certain
least and greatest fixed points collapse. I say "suggests", since proving that
these types are equivalent requires a parametric model, which I haven't actually
constructed yet. It also means that just polymorphism and the stability modality
□A are enough to construct an interesting reactive language, even
without a term-level fixed point fix x. e.

This function, minus the temporal staging, should also work for F plus
ordinary general recursion. I had just never thought about how the
coincidence plays out syntactically when you don't take recursive types as a
primitive!

This looks very interesting, since it looks like he's found a different way of enforcing causality than the one Nick and I hit on. We formalized the order structure of time (this happens before that), and required functions to respect that order. Alan has a discrete model of time (i.e., he doesn't bake temporal order into his category of times), and instead he uses parametricity to prove that all the definable functions (of the appropriate type) are inherently causal.

He's also been looking at using full temporal logic for typing reactive programs. This paper looks especially focused on the distinction between strong (must eventually happen) and weak eventually (may eventually happen) operators.

Brigitte Pientka visited us here at MPI last week, and she told me about some work that some of her students (Francisco Ferreira and someone whose name I unfortunately cannot recall -- Andrew Cave?) have been doing with her and Prakash Panangaden, on giving a proof theory for the modal mu-calculus. At a high level, their calculus looks very similar to the system I just wrote up, but there are some very intriguing semantic differences.

I use a Nakano-style guarded recursive type, which allows negative occurences. This permits defining general fixed points $(\bullet{A} \to A) \to A$, which collapses greatest and least fixed points. They use strict positivity instead, and so they can distinguish them, at the cost of more restrictive typing of recursive definitions. (This ties into Jeltsch's work on strong/weak eventually, which correspond to inductive versus coinductive definitions of the eventually type.)

Limit-colimit coincidence is one of the most striking features of categories of pointed domains, and it's (a) interesting that it can be induced already with guarded recursion, and (b) it might be easier to study in this setting.

The basic idea, roughly speaking, is to start with Mealy automata as a model of reactive programs. These automata lack a notion of higher-order behavior, but if you hit them with the Geometry of Interaction construction, you can get out a higher-order (linearly-typed) programming language, which is still implementable with finite state. This is a very nice idea, and reminiscent of Dan Ghica's work on synthesizing circuits.

Wednesday, November 14, 2012

Functional reactive programming (FRP) is an elegant approach
to declaratively specify reactive systems. However, the
powerful abstractions of FRP have historically made it
difficult to predict and control the resource usage of
programs written in this style.

In this paper we give a simple type theory for higher-order
functional reactive programming, as well as a natural
implementation strategy for it. Our type theory simplifies and
generalizes prior type systems for reactive programming. At
the same time, we give a an efficient implementation strategy
which eagerly deallocates old values, ruling out space
and time leaks, two notorious sources of inefficiency
in reactive programs. Our language neither restricts the
expressive power of the FRP model, nor does it require a
complex substructural type system to track the resource usage
of programs.

We also show that for programs well-typed under our type
system, our implementation strategy of eager deallocation is
safe: we show the soundness of our type system under our
implementation strategy, using a novel step-indexed Kripke
logical relation.

Last year, Nick Benton, Jan Hoffmann, and I worked out how to control the space usage of FRP programs. Our techniques worked, but it required a rather complicated linear type discipline to make everything work. In this paper, I've worked out how to dramatically simplify the type system, make the language more expressive, use simpler implementation strategies, which also make it more efficient.

This is also basically the implementation strategy I am using in my AdjS compiler, which should see a release as soon as I add support for a few more type system features, and squash the soundness bugs the correctness proof in this paper revealed.

Thursday, August 23, 2012

These days, most people are familiar with using monads to structure functional programs, and are also familiar with Haskell's do-notation for writing monadic code. Slightly less well-known is the fact that the do-notation actually originates in Moggi's original 1991 paper on monads, in which he gave the following type-theoretic introduction and elimination rules for monadic types:

Wednesday, August 8, 2012

In this post, I'll give an OCaml implementation of the pattern match compiler from my previous post. Just for fun,
it will also pattern compilation into a series of tests.

We begin by giving some some basic types. The type var
of variables is just strings, and we have a type tp of
types, which are just units, sums, products, void types, and a default
Other to handle all non-matchable types (e.g.,
functions). The type pat is the type of patterns. We have
patterns for variables, wildcards, units, pairs, and left and right
injections.

Finally, we have the type exp of expressions. We've thrown in
the minimum number of constructs necessary to elaborate pattern matching. Namely,
we have variables, let-binding, unit and pair elimination (both given in the "open-scope"
style), an abort (for the empty type), and a case expression for sums.

First, we give some utility operations. gensym does fresh name generation. This is an effect, but a simple one, and should be easy enough to transcribe into (say) monadic Haskell.

letgensym=letr=ref 0 infun()-> incr r;Printf.sprintf "u%d"!r

The filtermap operation maps an optional function over a list, returning the non-None values. I really should work out the general story for this someday -- there's obviously a nice story involving distributive laws at work here.

Unlike the simplify_unit and simplify_pair routines, the simplification routines for sums return
an option, which we filtermap over. This is because when we want the inl patterns, the inr
patterns are well-typed, but not needed. So we need to filter them out, but it's not actually an error to see them in the
branch list.